Question

Using just list_to_atom() gives:

list_to_atom("hello"). 
hello
list_to_atom("Hello").
'Hello'

why the difference?

I am trying to format a string with numbers, strings and atoms as follows:

lists:flatten(io_lib:format("PUTVALUE ~p ~p", [list_to_atom("hello"), 40])).
"PUTVALUE hello 40"
lists:flatten(io_lib:format("PUTVALUE ~p ~p", [list_to_atom("Hello"), 40])).
"PUTVALUE 'Hello' 40"

what is the best way of doing this in Erlang?

Edit: To make the question clear, there are more values than the example above and in some cases the value can be a string or an atom, like

lists:flatten(io_lib:format("PUTVALUE ~p ~p ~p", [list_to_atom("hello"), X, 40])).

where the first parameter is always a string but X can either be an atom or a string. The third parameter is always a number.

Answer 1

If you want to get a flat list for strings and integers, using ~s and ~B may be straitforward:

lists:flatten(io_lib:format("PUTVALUE ~s ~B", ["Hello", 40])).